Batch cooling crystallization and pressure filtration of sulphathiazole: the influence of solvent composition

Currently there is a great interest in new process analytical approaches to increase the process understanding of pharmaceutical unit operations. In the present study, the influence of the solvent composition on the material properties and, further, on the filtration characteristics, of different crystal suspensions obtained through an unseeded batch‐cooling‐crystallization process was studied. Sulphathiazole, which is an antibiotic agent with multiple polymorphic forms, was produced by performing laboratory‐scale cooling crystallization experiments from five different mixtures of water and propan‐1‐ol (n‐propanol). The size, shape and polymorphic composition of the crystals produced were characterized with a scanning electron microscope, with a novel automated image analyser and with an X‐ray powder diffractometer. All of the monitored crystal properties were found to clearly differ between the samples obtained from different solvents. The crystals produced in the batch‐cooling‐crystallization experiments were separated from the crystallizing solvents using a batch‐type pressure Nutsche filter, and the filtration characteristics of the suspensions were evaluated on the basis of average filter‐cake porosities and average specific cake resistances, which were determined from the experimentally obtained filtration data. Comparison between the calculated filtration characteristics revealed that considerable differences existed between the different suspensions, and it could therefore be concluded that the pressure‐filtration process was influenced by the composition of the crystallizing solvent. The filterability of all the studied sulphathiazole suspensions was considered to be rather good on the basis of the relatively low cake porosities (0.51–0.63), which were accompanied with low average specific cake resistances [(8.7×107)–(1.2×109) m/kg].